Water-based ink for ink-jet recording

ABSTRACT

A water-based ink for inkjet printing comprising a water dispersion of vinyl polymer particles prepared by containing a pigment in a vinyl polymer prepared by copolymerizing a monomer mixture comprising (a) a salt-forming group-containing monomer, (b) a macromer, and (c) a monomer copolymerizable with the salt-forming group-containing monomer and the macromer; and a process for preparing a water-based ink for inkjet printing comprising a water dispersion of vinyl polymer particles prepared by containing a pigment in a vinyl polymer, comprising dissolving in an organic solvent a vinyl polymer prepared by copolymerizing a monomer mixture comprising (a) a salt-forming group-containing monomer, (b) a macromer, and (c) a monomer copolymerizable with the salt-forming group-containing monomer and the macromer; adding a pigment to the resulting solution; pre-kneading the mixture; thereafter adding a neutralizing agent and water and kneading the mixture, to give an oil-in-water dispersion; and distilling off the organic solvent from the resulting kneaded product.

TECHNICAL FIELD

The present invention relates to a water-based ink for inkjet printing.

BACKGROUND ART

Inkjet printing system is a printing system comprising directly jettingink droplets from very fine nozzles, and depositing the ink droplets ona recording medium to give characters and images. This system hasadvantages that not only the used device shows a low level of noise andhas excellent operability, but also the coloration is facilitated andplain paper can be used as a recording medium. Therefore, this systemhas been widely used in the recent years.

In an ink used for inkjet printers, water-soluble dyes and polyhydricalcohols are used in order to prevent the ink from being clogged in thenozzles. However, this ink is poor in water resistance and lightfastness. Especially, when the ink is used as an ink for thermal jetsystem, there arise defects such that the dye is oxidized due to theheat of a heater surface, and that the ink is easily scorched on theheater surface, thereby lowering the discharging ability.

In order to eliminate this defect, there has been proposed an inkcomprising a water dispersion of a vinyl polymer having a specifiedstructure in which a hydrophobic dye is incorporated (Japanese PatentLaid-Open Nos. Hei 9-241565 and Hei 9-286939). However, this ink hasdefects that light fastness and printing density are insufficientbecause dye is used.

An object of the present invention is to provide a water-based ink forinkjet printing which is excellent in light fastness, imparts highprinting density, gives a recording medium no distortions, and isexcellent in water resistance, rub resistance, and highlighter-fastness.

DISCLOSURE OF INVENTION

According to the present invention, there are provided:

-   (1) a water-based ink for inkjet printing comprising a water    dispersion of vinyl polymer particles prepared by containing a    pigment in a vinyl polymer prepared by copolymerizing a monomer    mixture comprising (a) a salt-forming group-containing monomer, (b)    a macromer, and (c) a monomer copolymerizable with the salt-forming    group-containing monomer and the macromer; and-   (2) a process for preparing a water-based ink for inkjet printing    comprising a water dispersion of vinyl polymer particles prepared by    containing a pigment in a vinyl polymer, comprising dissolving in an    organic solvent a vinyl polymer prepared by copolymerizing a monomer    mixture comprising (a) a salt-forming group-containing monomer, (b)    a macromer, and (c) a monomer copolymerizable with the salt-forming    group-containing monomer and the macromer; adding a pigment to the    resulting solution; pre-kneading the mixture; thereafter adding a    neutralizing agent and water and kneading the mixture, to prepare a    water-in-oil dispersion; and distilling off the organic solvent from    the resulting kneaded product.

BEST MODE FOR CARRYING OUT THE INVENTION

The monomer mixture comprises (a) a salt-forming group-containingmonomer [hereinafter referred to as component (a)], (b) a macromer[hereinafter referred to as component (b)], and (c) a monomercopolymerizable with component (a) and component (b) [hereinafterreferred to as component (c)].

The component (a) includes cationic monomers and anionic monomers.Examples thereof include those listed in Japanese Patent Laid-Open No.Hei 9-286939, page 5, column 7, line 24 to column 8, line 29, and thelike.

Representative examples of the cationic monomers include unsaturatedtertiary amine-containing monomers, unsaturated ammonium salt-containingmonomers, and the like. Among them, N,N-dimethylaminoethyl(meth)acrylate, N-(N′,N′-dimethylaminopropyl) (meth)acrylamide andvinylpyrrolidone are preferable.

Representative examples of the anionic monomers include unsaturatedcarboxylic acid monomers, unsaturated sulfonic acid monomers,unsaturated phosphoric acid monomers, and the like. Among them,unsaturated carboxylic acid monomers such as acrylic acid andmethacrylic acid are preferable.

The component (b) includes macromers, which are monomers having apolymerizable unsaturated group, and a number-average molecular weightof 500 to 100000, preferably 1000 to 10000. Among them, siliconemacromers represented by the formula (II):X(Y)_(q)Si(R⁴)_(3-r)(Z)_(r)  (II)wherein X is a polymerizable unsaturated group; Y is a divalent bondgroup; each of R⁴ is independently hydrogen atom, a lower alkyl group,an aryl group or an alkoxy group; Z is a monovalent siloxane polymerresidue having a number-average molecular weight of not less than 500; qis 0 or 1; and r is an integer of 1 to 3; and/orstyrene macromers having a polymerizable functional group at one end arepreferable.

The number-average molecular weight of the component (b) is determinedby gel chromatography using 1 mmol/L of dodecyldimethylamine-containingchloroform as a solvent with polystyrene as a standard substance.

The silicone macromer can be favorably used from the viewpoint ofpreventing the head of an inkjet printer from scorching.

In the silicone macromers represented by the formula (II), X includesmonovalent unsaturated hydrocarbon groups having 2 to 6 carbon atomssuch as CH₂═CH— group and CH₂═C(CH₃)— group. Y includes divalent bondgroups such as —COO— group, —COOC_(a)H_(2a)— group (a being an integerof 1 to 5) and phenylene group, and —COOC₃H₆— is preferable. R⁴ includeshydrogen atom; lower alkyl groups having 1 to 5 carbon atoms such asmethyl group and ethyl group; aryl groups having 6 to 20 carbon atomssuch as phenyl group; and alkoxy groups having 1 to 20 carbon atoms suchas methoxy group. Among them, methyl group is preferable. Z ispreferably a monovalent residue of dimethylsiloxane polymer having anumber-average molecular weight of 500 to 5000. q is 0 or 1,preferably 1. r is an integer of 1 to 3, preferably 1.

Representative examples of the silicone macromers include siliconemacromers represented by the formula (II-1):CH₂═CR⁵—COOC₃H₆—[Si(R⁶)₂—O]_(b—Si(R) ⁶)₃  (II-1)wherein R⁵ is hydrogen atom or methyl group; each of R⁶'s isindependently hydrogen atom, or a lower alkyl group having 1 to 5 carbonatoms; b is a number of5 to 60;silicone macromers represented by the formula (II-2):CH₂═CR⁵—COO—[Si(R⁶)₂—O]_(b)—Si(R⁶)₃  (II-2)wherein R⁵, R⁶ and b are as defined above;silicone macromers represented by the formula (II-3):CH₂═CR⁵—Ph—[Si(R⁶)₂—O]_(b)—Si(R⁶)₃  (II-3)wherein Ph is phenylene group; and R⁵, R⁶ and b are as defined above;silicone macromers represented by the formula (II-4):CH₂═CR⁵—COOC₃H₆—Si(OE)₃  (II-4)wherein R⁵ is as defined above; and E is a —[Si(R⁵)₂O]_(c—Si(R) ⁵)₃group, wherein R⁵ is as defined above, and c is a number of 5 to 65, andthe like.

Among them, the silicone macromers represented by the formula (II-1) arepreferable, and especially preferable are silicone macromers representedby the formula (II-1a):CH₂═C(CH₃)—COOC₃H₆—[Si(CH₃)₂—O]_(d)— CH₃  (II-1a)wherein d is a number of 8 to 40. An example thereof includes FM-0711(trade name, manufactured by CHISSO CORPORATION), and the like.

The styrene macromers can be favorably used from the viewpoint ofsufficiently containing the pigment in the vinyl polymer.

The styrene macromers include styrene homopolymers having apolymerizable functional group at one end, or copolymers of styrene andother monomers. Among them, those having acryloyloxy group ormethacryloyloxy group as a polymerizable functional group at one end arepreferable. It is desired that the styrene content in the abovecopolymer is not less than 60% by weight, preferably not less than 70%by weight, from the viewpoint for sufficiently containing the pigment inthe vinyl polymer. The above other monomers include acrylonitrile, andthe like.

The component (c) includes (meth)acrylic acid esters such as methyl(meth)acrylate, ethyl (meth)acrylate, (iso)propyl (meth)acrylate, (iso-or tertiary-)butyl (meth)acrylate, (iso)amyl (meth)acrylate, cyclohexyl(meth)acrylate, benzyl (meth)acrylate, 2-ethylhexyl (meth)acrylate,(iso)octyl (meth)acrylate, (iso)decyl (meth)acrylate, (iso)dodecyl(meth)acrylate and (iso)stearyl (meth)acrylate; styrenic monomers suchas styrene, vinyltoluene, 2-methylstyrene and chlorostyrene, and thelike, each of which can be used alone or in admixture of at least twokinds. The above terms (iso- or tertiary-) and (meth) mean both caseswhere these groups are present and where they are absent. When thesegroups are absent, the group means a normal- group.

Incidentally, it is preferable that the component (c) contains thestyrenic monomer, from the viewpoints of improvements in printingdensity and highlighter-fastness. As the styrenic monomers, styrene and2-methylstyrene are preferable, and these may be used alone or incombination. In this case, it is desired that the content of thestyrenic monomers in the component (c) is 10 to 100% by weight,preferably 40 to 100% by weight, from the viewpoints of improvements inprinting density and highlighter-fastness.

In addition, the monomer mixture may contain at least one memberselected from the group consisting of (d) a hydroxyl group-containingmonomer [hereinafter referred to as “component (d)”], and (e) a monomerrepresented by the formula (I):CH₂═C(R¹)COO(R²O)_(p)R³  (I)wherein R¹ is hydrogen atom or a lower alkyl group; R² is a divalenthydrocarbon group having 1 to 30 carbon atoms, which may have ahetero-atom; R³ is a monovalent hydrocarbon group having 1 to 30 carbonatoms, which may have a hetero-atom; and p is a number of 1 to 60,[hereinafter referred to as “component (e)”].

The component (d) includes 2-hydroxyethyl (meth)acrylate,3-hydroxypropyl (meth)acrylate, polyethylene glycol(n=2 to 30)(meth)acrylate, poly(ethylene glycol(n=1 to 15)●propylene glycol(n=1 to15)) (meth)acrylate, and the like. Among them, 2-hydroxyethyl(meth)acrylate is preferable.

The component (e) exhibits excellent effects of enhancing dischargingstability of the water-based ink of the present invention, andsuppressing generation of wrinkles even when subjected to continuousprinting.

In the formula (I), R¹ is hydrogen atom or a lower alkyl group. Thelower alkyl group includes an alkyl group having 1 to 4 carbon atoms.

R² is a divalent hydrocarbon group having 1 to 30 carbon atoms, whichmay have a hetero-atom. The hetero-atom includes, for instance, nitrogenatom, oxygen atom, a halogen atom and sulfur atom.

Representative examples of R² include an aromatic ring having 6 to 30carbon atoms which may have a substituent; a heterocyclic ring having 3to 30 carbon atoms which may have a substituent; an alkylene grouphaving 1 to 30 carbon atoms which may have a substituent, and theserings or groups may be a combination of at least two kinds. Thesubstituent includes an aromatic ring having 6 to 29 carbon atoms; aheterocyclic ring having 3 to 29 carbon atoms; an alkyl group having 1to 29 carbon atoms; a halogen atom; amino group, and the like.

Suitable examples of R² include phenylene group which may have asubstituent having 1 to 24 carbon atoms; an aliphatic alkylene grouphaving 1 to 30 carbon atoms, preferably 1 to 20 carbon atoms; analkylene group having 7 to 30 carbon atoms having an aromatic ring; andan alkylene group having 4 to 30 carbon atoms having a heterocyclicring.

In addition, preferable examples of R²O group include ethylene oxidegroup, (iso)propylene oxide group, tetramethylene oxide group,heptamethylene oxide group, hexamethylene oxide group, and an alkyleneoxide group having 2 to 7 carbon atoms composed of combinations of atleast one of these alkylene oxides, and phenylene oxide group.

R³ is a monovalent hydrocarbon group having 1 to 30 carbon atoms, whichmay have a hetero-atom. The hetero-atom includes, for instance, nitrogenatom, oxygen atom and sulfur atom.

Representative examples of R³ include an aromatic ring having 6 to 30carbon atoms which may have a substituent; a heterocyclic ring having 3to 30 carbon atoms which may have a substituent; or an alkyl grouphaving 1 to 30 carbon atoms which may have a substituent. Thesubstituent includes an aromatic ring having 6 to 29 carbon atoms; aheterocyclic ring having 4 to 29 carbon atoms; a halogen atom; aminogroup, and the like.

Suitable examples of R³ include phenyl group; an aliphatic alkyl grouphaving 1 to 30 carbon atoms, preferably 1 to 20 carbon atoms; an alkylgroup having 7 to 30 carbon atoms having an aromatic ring; and an alkylgroup having 4 to 30 carbon atoms having a heterocyclic ring.

More preferable examples of R³ include alkyl groups having 1 to 6 carbonatoms such as methyl group, ethyl group, (iso)propyl group, (iso)butylgroup, (iso)pentyl group and (iso)hexyl group, phenyl group, and thelike.

p is a number of 1 to 60, and especially a number of 1 to 30 ispreferable.

Concrete examples of the component (e) include methoxypolyethyleneglycol(1 to 30: showing the value for p in the formula (I), hereinafterreferred to the same) (meth)acrylate, methoxypolytetramethylene glycol(1to 30) (meth)acrylate, ethoxypolyethylene glycol(1 to 30)(meth)acrylate, (iso)propoxypolyethylene glycol(1 to 30) (meth)acrylate,butoxypolyethylene glycol(1 to 30) (meth)acrylate, methoxypolypropyleneglycol(1 to 30) (meth)acrylate, methoxypoly(ethylene glycol●propyleneglycol)(1 to 30, in which ethylene glycol moiety is 1 to 29)(meth)acrylate, and the like. Each of these can be used alone or inadmixture of at least two kinds. Among them, methoxypolyethyleneglycol(1 to 30) (meth)acrylate is preferable. The term “(meth)acylate”in the present specification means acrylate or methacrylate. Inaddition, the term “(iso)propoxy” means n-propoxy or isopropoxy.

It is desired that the content of the component (a) in the vinyl polymeris 1 to 50% by weight, preferably 2 to 40% by weight, from the viewpointof the dispersion stability of the resulting dispersion.

It is desired that the content of the component (b) in the vinyl polymeris 1 to 25% by weight, preferably 5 to 20% by weight, from theviewpoints of suppression of scorching on the heater surface of theinkjet printer and stability.

It is desired that the content of the component (c) in the vinyl polymeris 5 to 93% by weight, preferably 10 to 80% by weight, from theviewpoints of suppression of scorching on the heater surface of theinkjet printer and stability. When the component (c) containing astyrenic monomer is used, it is preferable that the content of thecomponent (c) in the vinyl polymer is 10 to 60% by weight.

It is desired that the content of the component (d) in the vinyl polymeris 5 to 40% by weight, preferably 7 to 20% by weight, from theviewpoints of discharging stability and printing density. In addition,it is desired that the total content of the component (a) and thecomponent (d) is 6 to 60% by weight, preferably 10 to 50% by weight,from the viewpoints of stability in water and water resistance.

It is desired that the content of the component (e) in the vinyl polymeris 5 to 50% by weight, preferably 10 to 40% by weight, from theviewpoints of discharging stability and dispersion stability.

In addition, it is preferable that a total content of the component (a)and the component (e) is 6 to 75% by weight, from the viewpoints ofdispersion stability in water and discharging stability.

In addition, it is desired that a total content of the component (a),the component (d) and the component (e) is 6 to 60% by weight,preferably 7 to 50% by weight, from the viewpoints of dispersionstability in water and discharging stability.

The vinyl polymer is prepared by copolymerizing the monomer mixture by aknown polymerization method such as bulk polymerization method, solutionpolymerization method, suspension polymerization method, or emulsionpolymerization method. Among these polymerization methods, the solutionpolymerization method is preferable.

The solvent used in the solution polymerization method is preferably apolar organic solvent, and a water-miscible organic solvent can also beused by mixing with water. The organic solvent includes, for instance,aliphatic alcohols having 1 to 3 carbon atoms such as methanol, ethanoland propanol; ketones such as acetone and methyl ethyl ketone; esterssuch as ethyl acetate, and the like. Among them, methanol, ethanol,acetone, methyl ethyl ketone or a liquid mixture of these solvents withwater are preferable.

During polymerization, a radical polymerization initiator can be used.As the radical polymerization initiator, azo compounds such as2,2′-azobisisobutyronitrile, 2,2′-azobis(2,4-dimethylvaleronitrile),dimethyl-2,2′-azobisbutyrate, 2,2′-azobis(2-methylvaleronitrile), and1,1 ′-azobis(1-cyclohexanecarbonitrile) are preferable. In addition,organic peroxides such as t-butyl peroxyoctoate, di-t-butyl peroxide anddibenzoyl oxide can also be used.

It is preferable that the amount of the polymerization initiator is0.001 to 5% by mol, especially 0.01 to 2% by mol, of the monomermixture.

During the polymerization, a polymerization chain transfer agent may befurther added. Concrete examples of the polymerization chain transferagent include mercaptans such as octyl mercaptan, n-dodecyl mercaptan,t-dodecyl mercaptan, n-tetradecyl mercaptan and mercaptoethanol;xanthogenndisulfides such as dimethyl xanthogenndisulfide anddiisopropyl xanthogenndisulfide; thiuram disulfides such as tetramethylthiuram disulfide and tetrabutyl thiuram disulfide; halogenatedhydrocarbons such as carbon tetrachloride and ethylene bromide;hydrocarbons such as pentaphenylethane; unsaturated cyclic hydrocarboncompounds such as acrolein, methacrolein, allyl alcohol, 2-ethylhexylthioglycolate, terpinolene, α-terpinene, γ-terpinene, diterpene,α-methylstyrene dimer, 9,10-dihydroanthracene, 1,4-dihydronaphthalene,indene and 1,4-cyclohexadiene; unsaturated heterocyclic compounds suchas 2,5-dihydrofuran, and the like. These can be used alone or incombination of at least two kinds.

Although the conditions for polymerizing the monomer mixture aredifferent depending upon the kinds of the radical polymerizationinitiators used, the monomers, and the solvent, the polymerizationtemperature is usually 30° to 100° C., preferably 50° to 80° C., and thepolymerization time period is 1 to 20 hours. In addition, it ispreferable that the polymerization atmosphere is an inert gas atmospheresuch as nitrogen gas.

After termination of the polymerization reaction, the copolymer can beisolated from the reaction solution by a known method such asre-precipitation and distilling off of the solvent. In addition, theresulting copolymer can be purified by the removal of unreacted monomersby the repeat of re-precipitation, membrane separation, chromatography,extraction method, or the like.

The weight-average molecular weight of the vinyl polymer (determined inaccordance with the method described in the subsequent PreparationExamples 1 to 11 given below) is preferably 3000 to 200000, morepreferably 5000 to 100000, from the viewpoints of discharging property,prevention of scorching onto the printer head, durability of ink afterprinting, and stability of the dispersion.

The pigment may be an inorganic pigment or an organic pigment. Inaddition, an extender can be used together therewith, as occasiondemands.

The inorganic pigment includes carbon black, metal oxides, metalsulfides, metal chlorides, and the like. Among them, especially amongthe black water-based ink, carbon black is preferable. The carbon blackincludes furnace black, thermal lamp black, acetylene black, channelblack, and the like.

The organic pigment includes azo pigments, diazo pigments,phthalocyanine pigments, quinacridone pigments, isoindolinone pigments,dioxazine pigments, perylene pigments, perinone pigments, thioindigopigments, anthraquinone pigments, quinophthalone pigments, and the like.

The extender includes silica, calcium carbonate, talc, and the like.

It is desired that the amount of the pigment is 20 to 400 parts byweight, preferably 50 to 300 parts by weight, based on 100 parts byweight of the resin solid ingredients of the vinyl polymer, from theviewpoint of printing density and facilitation in containing the pigmentin the vinyl polymer.

The water dispersion of the vinyl polymer particles containing thepigment can be obtained by, for instance, dissolving a vinyl polymer inan organic solvent; adding a pigment to the resulting solution;pre-kneading the mixture; thereafter adding a neutralizing agent andwater and kneading the resulting mixture, to prepare an oil-in-watertype dispersion; and distilling off the organic solvent from the kneadedmixture.

The organic solvent is preferably an alcohol solvent, a ketone solventand an ether solvent, and a hydrophilic organic solvent is morepreferable.

The alcohol solvent includes methanol, ethanol, isopropanol, n-butanol,tertiary butanol, isobutanol, diacetone alcohol, and the like. Theketone solvent includes acetone, methyl ethyl ketone, diethyl ketone,methyl isobutyl ketone, and the like. The ether solvent includes dibutylether, tetrahydrofuran, dioxane, and the like. Among these solvents,isopropanol, acetone and methyl ethyl ketone are preferable.

In addition, the above organic solvent may be used together with ahigh-boiling point hydrophilic organic solvent, as occasion demands. Thehigh-boiling point hydrophilic organic solvent includes phenoxyethanol,ethylene glycol monomethyl ether, diethylene glycol monomethyl ether,diethylene glycol monoethyl ether, diethylene glycol diethyl ether, andthe like.

As the neutralizing agent, an acid or base can be used depending uponthe kinds of the salt-forming groups. The acid includes inorganic acidssuch as hydrochloric acid and sulfuric acid; and organic acids such asacetic acid, propionic acid, lactic acid, succinic acid, glycolic acid,gluconic acid and glyceric acid. The base includes tertiary amines suchas trimethylamine and triethylamine, ammonia, sodium hydroxide,potassium hydroxide, and the like.

The degree of neutralization is not limited to specified ones. It ispreferable that the resulting water dispersion is usually neutral, forinstance, pH is 4.5 to 9.

The particle diameter of the vinyl polymer particles containing thepigment is preferably 0.01 to 0.50 μm, more preferably 0.02 to 0.20 μm,from the viewpoint of prevention of clogging of the nozzles and thedispersion stability.

It is desired that the content of the water dispersion of the vinylpolymer in the water-based ink is adjusted such that the content of thevinyl polymer particles in the water-based ink is usually 1 to 30% byweight, preferably 2 to 15% by weight, from the viewpoints of printingdensity and discharging stability.

There can be added additives such as wetting-agents, dispersing agents,defoaming agents, mildewproof agents and chelating agents to thewater-based ink for inkjet printing of the present invention.

As the wetting agents, there can be used polyhydric alcohols and ethersthereof such as ethylene glycol, propylene glycol, diethylene glycol,triethylene glycol, polyethylene glycols, glycerol, diethylene glycoldiethyl ether, and diethylene glycerol mono-n-butyl ether; acetates;nitrogen-containing compounds such as N-methyl-2-pyrrolidone and1,3-dimethylimidazolidinone. The content of the wetting agent in thewater-based ink is preferably 0.1 to 50% by weight, more preferably 0.1to 30% by weight.

In addition, as the dispersing agent, there can be used anionic,nonionic, cationic and amphoteric dispersing agents.

Also, as the defoaming agent, it is especially preferable to use thecompound represented by the formula (III):

wherein each of R⁷, R⁸, R⁹ and R¹⁰ is independently an alkyl grouphaving 1 to 10 carbon atoms, or phenyl group which may have asubstituent; each of R¹¹ and R¹² is independently an alkyl group having1 to 10 carbon atoms, phenyl group which may have a substituent,hydroxyl group, amino group, carboxyl group or epoxy group,respectively; each of x and y is independently a number of 0 to 1000,preferably 10 to 100,from the viewpoints of suppression of generation of foams during thepreparation of the water-based ink and adjustment of surface tension ofthe water-based ink.

The content of the defoaming agent in the water-based ink is preferably0.001 to 2.0% by weight, more preferably 0.005 to 0.5% by weight.

PREPARATION EXAMPLES 1 to 3

A reaction vessel was charged with 20 parts by weight of methyl ethylketone, monomers and a polymerization chain transfer agent which arelisted in the column of “initially charged monomers” of Table 1, andnitrogen gas substitution was sufficiently carried out.

On the other hand, a dropping funnel was charged with monomers and apolymerization chain transfer agent which are listed in the column of“dropping monomers” of Table 1, 60 parts by weight of methyl ethylketone and 1.2 parts by weight of2,2′-azobis(2,4-dimethylvaleronitrile), and nitrogen gas substitutionwas sufficiently carried out.

The mixed solution inside the reaction vessel was heated to 65° C. withstirring under nitrogen atmosphere, and the mixed solution inside thedropping funnel was slowly added dropwise thereto over a period of 3hours. After 2 hours passed from the termination of dropping, theretowas added a solution prepared by adding 0.1 parts by weight of2,2′-azobis(2,4-dimethylvaleronitrile) to 5 parts by weight of methylethyl ketone. The mixture was further aged at 65° C. for 2 hours andthen at 70° C. for 2 hours, to give a copolymer solution.

A part of the resulting copolymer solution was isolated by drying it at105° C. for 2 hours under reduced pressure to completely remove thesolvent. The weight-average molecular weight was determined by gelpermeation chromatography using polystyrene as a standard substance, andchloroform containing dodecyldimethylamine of 1 mmol/L as a solvent. Asa result, each of the resulting copolymers had a weight-averagemolecular weight of about 10000.

Five parts by weight of the copolymer obtained by drying the resultingcopolymer solution under reduced pressure was dissolved in 25 parts byweight of an organic solvent listed in Table 1, and 2 parts by weight ofa neutralizing agent (30% aqueous solution) listed in Table 1 was addedthereto to neutralize a part of the copolymer. Further, 30 parts byweight of ion-exchanged water and 5 parts by weight of a pigment wereadded thereto, and the mixture was kneaded with a beads mill.

The organic solvent was completely removed from the resulting kneadedproduct at 60° C. under reduced pressure, and a part of water wasfurther removed, thereby giving a water dispersion of pigment-containingvinyl polymer particles, the concentration of the solid ingredients ofwhich was 20% by weight.

PREPARATION EXAMPLE 4

A copolymer solution was obtained in the same manner as in PreparationExample 1 using the monomers and a polymerization chain transfer agentlisted in Table 1. The weight-average molecular weight of the resultingcopolymer was obtained in the same manner as in Preparation Example 1.As a result, it was found to be about 10000.

Twenty-five parts by weight of an organic solvent listed in Table 1 and5 parts by weight of a dye or pigment were added to 5 parts by weight ofthe copolymer obtained by drying the resulting copolymer solution underreduced pressure, and completely dissolved. Two parts by weight of aneutralizing agent (30% aqueous solution) listed in Table 1 was addedthereto to neutralize a part of the salt-forming group of the copolymer.Further, 300 parts by weight of ion-exchanged water was added thereto,and the mixture was stirred. Thereafter, the mixture was emulsified for30 minutes by using a microfluidizer (manufactured by MicrofluidicsInternational Corporation).

The organic solvent was completely removed from the resulting emulsionat 60° C. under reduced pressure, and water was further removed toconcentrate the mixture, thereby giving a water dispersion ofdye-containing vinyl polymer particles, the concentration of the solidingredients of which was 20% by weight.

The names listed in Tables 1 and 3 mean the followings.

-   Silicone Macromer: Manufactured by Chisso Corporation under the    trade name: FM-0711 (number-average molecular weight: 1000, a    compound represented by the formula (II-1a))-   Pigment Red 122: Manufactured by Toyo Ink Manufacturing Co., Ltd.,    under the trade name: LIONOGEN Magenta R-   Pigment Yellow 17: Manufactured by DAINICHISEIKA COLOR & CHEMICALS    MFG. CO., LTD., under the trade name: ECY-215-   Carbon Black: Manufactured by Degussa, under the trade name:    Printex-90-   Spilon Red: Manufactured by Hodogaya Chemical Co., Ltd., under the    trade name: Spilon Red C-BR-   Styrene Macromer A: Manufactured by TOAGOSEI CO., LTD., under the    trade name: AN-6 (styrene-acrylonitrile macromer, styrene content:    75% by weight, number-average molecular weight: 6000, polymerizable    functional group: methacroyloxy group)-   Styrene Macromer B: Manufactured by TOAGOSEI CO., LTD., under the    trade name: AS-6 (styrene macromer, number-average molecular weight:    6000, polymerizable functional group: methacroyloxy group)

TABLE 1 Preparation Examples 1 2 3 4 Initially Charged Monomers Styrene[10] t-Butyl Methacrylate [20] Styrene [10] t-Butyl Methacrylate [20]2-Ethylhexyl Acrylate [17] Polyethylene Glycol n-Butyl Acrylate [10]2-Hydroxyethyl 2-Hydroxyethyl Methacrylate Methacrylate [10]2-Hydroxyethyl Acrylate [5] Methacrylate [5] [10] Silicone Macromer [2]Polyethylene Glycol Silicone Macromer [5] Silicone Macromer [2]Methacrylic Acid [4] Methacrylate [5] Acrylic Acid [10] Acrylic Acid [3]n-Dodecylmercaptan [0.6] Silicone Macromer [2] t-Dodecylmercaptan [0.6]n-Dodecylmercaptan [0.6] Dimethylaminoethyl Acrylamide [10]n-Hexylmercaptan [0.5] Dropping Monomers Styrene [10] t-ButylMethacrylate [32] Styrene [10] t-Butyl Methacrylate [22] 2-EthylhexylAcrylate [25] Polyethylene Glycol n-Butyl Acrylate [14] 2-HydroxyethylMethacrylate 2-Hydroxyethyl Methacrylate Methacrylate [15]2-Hydroxyethyl Acrylate [10] [10] [10] Silicone Macromer [8]Polyethylene Glycol Silicone Macromer [10] Silicone Macromer [8]Methacrylic Acid [6] Methacrylate [5] Acrylic Acid [15] Acrylic Acid [2]n-Dodecylmercaptan [2.4] Silicone Macromer [6] t-Dodecylmercaptan [2.4]n-Dodecylmercaptan [2.4] Dimethylaminoethyl Acrylamide [10.5]n-Hexylmercaptan [2.0] Pigment Pigment Red 122 Pigment Yellow 17 CarbonBlack Spilon Red (Dye) Organic Solvent Methyl Ethyl Ketone AcetoneMethyl Ethyl Ketone Acetone Neutralizing Agent Sodium Hydroxide AmmoniaGlycolic Acid Sodium Hydroxide (Note): Amounts of each ingredient insideparenthesis indicate parts by weight.

EXAMPLES 1 to 3

Forty parts by weight of the water dispersion of pigment-containingvinyl polymer particles obtained in Preparation Examples 1 to 3, 5 partsby weight of glycerol, 10 parts by weight of urea, 1 part by weight ofacetylene glycol-polyethylene oxide adduct (manufactured by Kawaken FineChemicals Co., Ltd., under the trade name: Acetylenol EH) and 44 partsby weight of ion-exchanged water were mixed, and the resulting liquidmixture was filtered with a needle-less syringe [manufactured by TERUMOCORPORATION] having a volume of 25 mL to which 0.5 μm filter [acetylcellulose membrane, outer diameter: 2.5 cm, manufactured by Fuji PhotoFilm Co., Ltd.] was attached, thereby removing coarse grains, to givewater-based inks of Examples 1 to 3 corresponding to PreparationExamples 1 to 3, respectively.

COMPARATIVE EXAMPLE 1

A water-based ink was obtained by the following formulation.

(Formulation) (Parts by Weight) C.I. Acid Red 52  4 (Water-Soluble Dye)Glycerol  5 Diethylene Glycol 10 Isopropyl Alcohol  2 Ion-ExchangedWater 79

COMPARATIVE EXAMPLE 2

A water-based ink was obtained in the same manner as in ComparativeExample 1, except for using 4 parts by weight of carbon black in placeof C. I. Acid Red 52, and dispersing by using a sodium salt of anaromatic sulfonic acid-formalin condensation product (manufactured byKao Corporation, under the trade name: Demole C).

COMPARATIVE EXAMPLE 3

A water-based ink was obtained in the same manner as in Example 1,except for using 40 parts by weight of a water dispersion of thedye-containing vinyl polymer particles obtained in Preparation Example 4in place of the water dispersion of the pigment-containing vinyl polymerparticles obtained in Preparation Example 1.

The physical properties for the water-based inks obtained in Examples 1to 3 and Comparative Examples 1 to 3 were evaluated by the followingmethods. The results are shown in Table 2.

(1) Printing Density

Solid image printing was carried out on commercially available copypaper using a commercially available bubble jet printer manufactured byHEWLETT PACKARD (Model Number: Desk Jet-720C), and the printed image wasallowed to stand at 25° C. for 1 hour, and thereafter the printingdensity was determined by using a Macbeth densitometer (manufactured byMacbeth Process Measurements Co., Product Number: RD914), and evaluatedby the following evaluation criteria:

[Evaluation Criteria]

-   ∘: Printing density being not less than 1.1-   Δ: Printing density being not less than 1.0 and less than 1.1-   x: Printing density being less than 1.0    (2) Light Fastness

The solid printed object for which the printing density was determinedabove was continued to be irradiated for 10000 counts with Xenon FadeMeter (trade name, manufactured by ATLAS). Thereafter, the printingdensity of the same printing portion determined before irradiation wasagain determined with Macbeth densitometer RD914. The residual ratio ofthe printing density after irradiation to the printing density beforeirradiation was calculated by the following equation:$\lbrack {{Residual}\quad{Ratio}} \rbrack = {\frac{\lbrack {{Printing}\quad{Density}\quad{After}\quad{Irradiation}} \rbrack}{\lbrack {{Printing}\quad{Density}\quad{Before}\quad{Irradiation}} \rbrack} \times 100}$The light fastness was evaluated on the basis of the followingevaluation criteria:[Evaluation Criteria]

-   ∘: Residual ratio being not less than 95%-   Δ: Residual ratio being not less than 80% and less than 95%-   x: Residual ratio being less than 80%    (3) Water Resistance

Solid image printing was carried out on a commercially available copypaper using the above-mentioned printer, and the printing density of aspecified printed portion of the sample which was dried at 25° C. for 1hour was determined. Thereafter, the printed paper was immersedvertically in stand-still water for 10 seconds, and then verticallypulled out therefrom in this state. The immersed paper was air-dried at25° C. for 24 hours, and the printing density of the same printingportion as determined before immersion was again determined. Theresidual ratio of the printing density after immersion to the printingdensity before immersion was calculated by the equation:$\lbrack {{Residual}\quad{Ratio}} \rbrack = {\frac{\lbrack {{Printing}\quad{Density}\quad{After}\quad{Immersion}} \rbrack}{\lbrack {{Printing}\quad{Density}\quad{Before}\quad{Immersion}} \rbrack} \times 100}$The water resistance was evaluated on the basis of the followingevaluation criteria:[Evaluation Criteria]

-   ⊚: Residual ratio being not less than 95%-   ∘: Residual ratio being not less than 90% and less than 95%-   Δ: Residual ratio being not less than 70% and less than 90%-   x: Residual ratio being less than 70%    (4) Rub Resistance

Solid image printing was carried out on a commercially available copypaper using the above-mentioned printer, and the printed surface wasstrongly rubbed with a finger. The extent of rub-off of the printedimage was evaluated by the following evaluation criteria:

[Evaluation Criteria]

-   ∘: Substantially no printed image being rubbed off, and its    periphery not being stained-   Δ: Some printed image being rubbed off, its periphery being slightly    stained, and finger also being slightly stained-   x: Printed image being considerably rubbed off, its periphery being    considerably stained, and finger also being considerably stained    (5) High Lighter-Fastness

Text printing was carried out on a commercially available copy paperusing the above-mentioned printer, and after passage of 6 hours at 25°C., the extent of staining of the printed sample when traced with acommercially available aqueous fluorescent marker was observed withnaked eyes, and the evaluation was made on the basis of the followingevaluation criteria:

[Evaluation Criteria]

-   ⊚: No staining such as rubbed stains was observed even when traced    with a fluorescent marker.-   ∘: Some rubbed stains were generated when traced with a fluorescent    marker, but at a level without practical problems.-   x: Generation of rubbed stains were observed when traced with a    fluorescent marker, which was intolerable.

TABLE 2 Physical Properties of Water-Based Ink Printing Light Water RubHigh Lighter- Density Fastness Resistance Resistance Fastness Ex. No. 1∘ ∘ ⊚ ∘ ∘ 2 ∘ ∘ ⊚ ∘ Δ 3 ∘ ∘ ⊚ ∘ ∘ Comp. Ex. 1 ∘ x x ∘ Δ 2 ∘ ∘ Δ x x 3 ∘x ⊚ ∘ ∘ (Note) In the table, the evaluation “x” means not usable.

It is clear from the results shown in Table 2 that all of thewater-based inks obtained in each example are excellent in lightfastness, and give high printing density, and further are excellent inwater resistance, rub resistance and highlighter-fastness.

PREPARATION EXAMPLES 5 to 11

A reaction vessel was charged with 20 parts by weight of an organicsolvent shown in Table 3, monomers and a polymerization chain transferagent which are listed in the column of “initially charged monomers” ofTable 3, and nitrogen gas substitution was sufficiently carried out.

On the other hand, a dropping funnel was charged with monomers and apolymerization chain transfer agent which are listed in the column of“dropping monomers” of Table 3, 60 parts by weight of methyl ethylketone, and 1.2 parts by weight of2,2′-azobis(2,4-dimethylvaleronitrile), and nitrogen gas substitutionwas sufficiently carried out.

The mixed solution inside the reaction vessel was heated to 65° C. withstirring under nitrogen atmosphere, and the mixed solution inside thedropping funnel was slowly added dropwise thereto over a period of 3hours. After 2 hours passed from the termination of dropping, theretowas added a solution prepared by adding 0.3 parts by weight of2,2′-azobis(2,4-dimethylvaleronitrile) to 5 parts by weight of methylethyl ketone. The mixture was further aged at 65° C. for 2 hours andthen at 70° C. for 2 hours, to give a copolymer solution.

A part of the resulting copolymer solution was isolated by drying it at105° C. for 2 hours under reduced pressure to completely remove thesolvent. The weight-average molecular weight was determined by gelpermeation chromatography using polystyrene as a standard substance, andchloroform containing dodecyldimethylamine of 1 mmol/L as a solvent. Theresults are shown in Table 3.

Five parts by weight of the copolymer obtained by drying the resultingcopolymer solution under reduced pressure was dissolved in 25 parts byweight of an organic solvent listed in Table 3, and a given amount of aneutralizing agent (30% aqueous solution) listed in Table 3 was addedthereto to neutralize a part of the copolymer. Further, 5 parts byweight of a pigment was added thereto, and the mixture was kneaded witha beads mill.

Thirty parts by weight of ion-exchanged water was added to the resultingkneaded product, and the mixture was stirred. Thereafter, the organicsolvent was removed from the resulting kneaded product at 60° C. underreduced pressure, and a part of water was further removed, therebygiving a water dispersion of pigment-containing vinyl polymer particles,the concentration of the solid ingredients of which was 20% by weight.

TABLE 3 Preparation Examples 5 6 7 8 Initially Charged Monomers t-ButylMethacrylate [18] Styrene [20] t-Butyl Methacrylate [20] Styrene [20]Acrylic Acid [2] Methacrylic Acid [4] N,N-DimethylaminoethylN,N-Dimethylaminoethyl Methoxypolytetramethylene ButoxypolyethyleneGlycol(9) Methacrylate [20] Acrylate [2] Glycol(12) Methacrylate [10]Methacrylate [10] Methoxypolyethylene Methoxypolyethylene StyreneMacromer A [5] Silicone Macromer [5] Glycol(9) Methacrylate [5]Glycol(23) Methacrylate [20] 2-Hydroxyethyl Methacrylate StyreneMacromer B [5] Silicone Macromer [2] Styrene Macromer A [5] [10]n-Dodecylmercaptan [0.3] n-Dodecylmercaptan [0.2] Mercaptoethanol [0.2]n-Dodecylmercaptan [0.3] Dropping Monomers t-Butyl Methacrylate [20]Styrene [25] t-Butyl Methacrylate [23] Styrene [23] Acrylic Acid [3]Methacrylic Acid [4] N,N-Dimethylaminoethyl N,N-DimethylaminoethylMethoxypolytetramethylene Butoxypolyethylene Glycol(9) Methacrylate [20]Acrylate [3] Glycol(12) Methacrylate [15] Methacrylate [15]Methoxypolyethylene Methoxypolyethylene Styrene Macromer A [5] SiliconeMacromer [5] Glycol(9) Methacrylate [5] Glycol(23) Methacrylate [20]2-Hydroxyethyl Methacrylate Styrene Macromer B [5] Silicone Macromer [3]Styrene Macromer A [5] [10] n-Dodecylmercaptan [1.7] n-Dodecylmercaptan[1.8] Mercaptoethanol [1.8] n-Dodecylmercaptan [1.7] Weight-AverageMolecule Weight 11000 38000 24000 21000 Pigment Pigment Red 122 PigmentYellow 17 Carbon Black Carbon Black Organic Solvent Methyl Ethyl KetoneAcetone Methyl Ethyl Ketone Methyl Ethyl Ketone Neutralizing AgentSodium Hydroxide [0.5] Ammonia [0.3] Glycolic Acid [3.2] Acetic Acid[0.4] 9 10 11 Initially Charged Monomers Styrene [7.5] Styrene [8]Styrene [15] n-Dodecyl Methacrylate [3] N,N-Dimethylaminoethyl MethylMethacrylate [4.5] N,N-Dimethylaminoethyl Methacrylate [15]N,N-Dimethylaminoethyl Methacrylate [10] Methoxypolyethylene Glycol(4)Methacrylate [25] Methoxypolyethylene Glycol(4) Methacrylate [15]Styrene Macromer B [5] Methacrylate [20] 2-Hydroxyethyl Methacrylate [2]Mercaptoethanol [0.2] Styrene Macromer B [5] Silicone Macromer [1]Mercaptoethanol [0.2] Styrene Macromer B [5] Mercaptoethanol [0.2]Dropping Monomers Styrene [7.5] Styrene [10] Styrene [15] n-DodecylMethacrylate [5] N,N-Dimethylaminoethyl Methyl Methacrylate [4.5]N,N-Dimethylaminoethyl Methacrylate [15] N,N-DimethylaminoethylMethacrylate [15] Methoxypolyethylene Glycol(4) Methacrylate [25]Methoxypolyethylene Glycol(4) Methacrylate [18] Styrene Macromer B [5]Methacrylate [20] 2-Hydroxyethyl Methacrylate [3] Mercaptoethanol [0.8]Styrene Macromer B [5] Silicone Macromer [1] Mercaptoethanol [1.8]Styrene Macromer B [5] Mercaptoethanol [1.8] Weight-Average MolecularWeight 15000 43000 37000 Pigment Carbon Black Carbon Black Carbon BlackOrganic Solvent Methyl Ethyl Ketone Methyl Ethyl Ketone Methyl EthylKetone Neutralizing Agent Gluconic Acid [5.2] Glyceric Acid [2.9]Gluconic Acid [10.4] (Note): Amounts of each ingredient insideparenthesis indicate parts by weight.

EXAMPLES 4 to 10

Forty parts by weight of the water dispersion of pigment-containingvinyl polymer particles obtained in Preparation Examples 5 to 11, 5parts by weight of glycerol, 10 parts by weight of urea, 1 part byweight of acetylene glycol-polyethylene oxide adduct (manufactured byKawaken Fine Chemicals Co., Ltd., under the trade name: Acetylenol EH)and 44 parts by weight of ion-exchanged water were mixed, and theresulting liquid mixture was filtered with a needle-less syringe[manufactured by TERUMO CORPORATION] having a volume of 25 mL to which0.5 μm filter (acetyl cellulose membrane, outer diameter: 2.5 cm,manufactured by Fuji Photo Film Co., Ltd.) was attached, therebyremoving coarse grains, to give water-based inks of Examples 4 to 10corresponding to Preparation Examples 5 to 11, respectively.

Printing density, light fastness, water resistance, rub resistance andhighlighter-fastness were evaluated for the water-based inks obtained inExamples 4 to 10 in the same manner as in Examples 1 to 3.

In addition, distortions of the recording medium were evaluated for thewater-based inks obtained in Examples 4 to 10 and the water-based inksobtained in Comparative Examples 1 to 3 in accordance with the followingmethod. The results are shown in Table 4.

(6) Distortion of Printed Objects

The extent of distortion when printing a ruled line using theabove-mentioned printer was observed with naked eyes, and evaluated bythe following evaluation criteria:

[Evaluation Criteria]

-   ⊚: No distortion-   ∘: Almost no distortion-   Δ: Some distortions-   x: Much distortions

TABLE 4 Physical Properties of Water-Based Ink Distortion Water Rub HighPrinting of Printed Light Resist- Resist- Lighter- Density ObjectFastness ance ance Fastness Ex. No. 4 ∘ ⊚ ∘ ⊚ ∘ ⊚ 5 ∘ ∘ ∘ ⊚ ∘ ∘ 6 ∘ ∘ ∘⊚ ∘ ⊚ 7 ∘ ⊚ ∘ ⊚ ∘ ∘ 8 ∘ ⊚ ∘ ⊚ ∘ ∘ 9 ∘ ∘ ∘ ⊚ ∘ ∘ 10  ∘ Δ ∘ ⊚ ∘ ∘ Comp.Ex. 1 ∘ ⊚ x x ∘ Δ 2 ∘ Δ ∘ Δ x x 3 ∘ x x ⊚ ∘ ∘ (Note) In the table, theevaluation “x” means not usable.

It is clear form the results shown in Table 4 that all of thewater-based inks obtained in each example are excellent in lightfastness, and give high printing density, with no distortions of theprinted objects, and are excellent in water resistance, rub resistanceand highlighter-fastness.

INDUSTRIAL APPLICABILITY

The water-based ink for inkjet printing of the present invention isexcellent in light fastness, gives high printing density, with nodistortion of printed objects, and further is excellent in waterresistance, rub resistance and highlighter-fastness.

1. A water-based ink for inkjet printing comprising a water dispersionof vinyl polymer particles prepared by containing a pigment in a vinylpolymer prepared by copolymerizing a monomer mixture comprising (a) asalt-forming group-containing monomer, (b) a styrene macromer having apolymerizable functional group at one end, and (c) at least one monomercopolymerizable with the salt-forming group-containing monomer and thestyrene macromer, which monomer is a styrenic monomer; and wherein thecontent of the styrenic monomer in the vinyl polymer is 10 to 60% byweight.
 2. The water-based ink according to claim 1, wherein the monomermixture comprises additionally at least one member selected from thegroup consisting of (d) hydroxyl group-containing monomers, and (e)monomers represented by the formula (I):CH₂═C(R¹)COO(R²O)_(p)R³  (I) wherein R¹ is hydrogen atom or a loweralkyl group; R² is a divalent hydrocarbon group having 1 to 30 carbonatoms, which may have a hetero-atom; R³ is a monovalent hydrocarbongroup having 1 to 30 carbon atoms, which may have a hetero-atom; and pis a number of 1 to
 60. 3. The water-based ink according to claim 1,wherein the weight-average molecular weight of the vinyl polymer is 3000to
 200000. 4. The water-based ink according to claim 1, wherein themonomer mixture comprises additionally a (meth)acrylic acid ester.
 5. Aprocess for preparing a water-based ink for inkjet printing comprising awater dispersion of vinyl polymer particles prepared by containing apigment in a vinyl polymer, comprising dissolving in an organic solventa vinyl polymer prepared by copolymerizing a monomer mixture comprising(a) a salt-forming group-containing monomer, (b) a styrene macromerhaving a polymerizable functional group at one end, and (c) at least onemonomer copolymerizable with the salt-forming group-containing monomerand the styrene macromer, which monomer is a styrenic monomer; adding apigment to the resulting solution; pre-kneading the mixture; thereafteradding a neutralizing agent and water, and kneading the mixture, to givean oil-in-water dispersion; and distilling off the organic solvent fromthe resulting kneaded product, wherein the content of the styrenicmonomer in the vinyl polymer is 10 to 60% by weight.
 6. The process forpreparing a water-based ink according to claim 5, wherein the monomermixture comprises additionally at least one member selected from thegroup consisting of (d) hydroxyl group-containing monomers, and (e)monomers represented by the formula (I):CH₂═C(R¹)COO(R²O)_(p)R³  (I) wherein R¹ is hydrogen atom or a loweralkyl group; R² is a divalent hydrocarbon group having 1 to 30 carbonatoms, which may have a hetero-atom; R³ is a monovalent hydrocarbongroup having 1 to 30 carbon atoms, which may have a hetero-atom; and pis a number of 1 to 60.